Overwintering Large Koi in Minnesota: 4,000 Gal, 100+ Fish

4,000-gal, 4-ft deep MN pond. 2 large koi (>12"), 3 small koi, many goldfish (~100 total). Deicer + pump spouting for aeration. Asks: risk of dying? Feed large koi?

Assessment

Volume is adequate for current biomass if filtration is solid. 4,000 gal handles 100 small fish + a few large ones if biofiltration processes waste.

Critical Actions

1. Feed large koi until water consistently <55°F (wheat-germ food). They need energy reserves.
2. Stop feeding all fish below 50°F.
3. Ensure gas exchange: Deicer maintains hole. Add aerator at mid-depth (not bottom) to avoid supercooling.
4. Monitor ammonia during warm spells—partial water change if detected.
5. Plan for spring: As fish grow, 4,000 gal will become undersized. Ammonia issues will emerge.

Current stocking is manageable for this winter. Long-term, reduce numbers or expand pond.